1. Field of the Invention
This invention relates to Bulk Current Injection (BCI) transformers and, more particularly, to BCI transformers with improved magnetic field uniformity, bandwidth and power handling.
2. Description of the Related Art
The following descriptions and examples are given as background only.
Bulk Current Injection (BCI) immunity test methods are often used to evaluate the electromagnetic susceptibility of a wide range of electronic devices. The BCI method injects continuous wave or pulsed current onto the electrical conductor(s) of a device under test (DUT) to mimic the excitation of common mode current on the conductors by far field (plane wave) electromagnetic radiation. BCI tests are desirable because, over a given frequency range, they typically require much less power to produce a specified common mode current on a conductor than a radiated immunity test.
BCI tests use “flux coupled” or Faraday transformers, called BCI probes, to inject large RF currents onto one or more conductors of an electronic device. A typical BCI probe 100 is depicted in FIG. 1. As shown in FIG. 1, BCI probe 100 includes a magnetic core 110, which encircles the conductor(s) 120 onto which common mode current is to be excited. A single magnetic coil 130, comprising one or more turns, is wrapped around the magnetic core at the input or “feed point” of the transformer.
In injection mode, magnetic coil 130 acts as a primary winding and conductors 120 of the DUT act as a secondary winding. RF power source 140 is connected to the primary winding. Current flow in the primary winding (the “primary current”) generates a magnetic flux in the core, which in turn, induces current to flow in the secondary winding via electromagnetic coupling. In addition to current injection, BCI transformers may be used to sense and measure common mode current on conductors 120. In sensing mode, conductors 120 act as the primary winding, and coil 130 wound around core 110 acts as the secondary winding. The transformer is called a “current monitor” when used to sense common mode current.
Recently, efforts have been made to characterize BCI transformers over broader frequency ranges as BCI tests are extended to increasingly higher frequencies. However, little published work exists concerning the electromagnetic fields produced by such devices. There is also little published work concerning the distributed or retarded electromagnetic effects (sometimes referred to as dimensional effects) within the magnetic cores of these transformers, or how these effects manifest themselves in the injected or sensed current. The present invention alleviates this oversight by providing a test fixture for characterizing the distributed effects produced within the magnetic cores of BCI transformers. Exemplary solutions are also provided herein for overcoming such effects.